mlxsw: spectrum_router: Prepare function to return errors

The function mlxsw_sp_router_fib6_event() takes care of preparing the
needed information for the work item that actually inserts the route
into the device.

When processing an IPv6 multipath route, the function will need to
allocate an array to store pointers to all the sibling routes.

Change the function's signature to return an error code and adjust the
single call site.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>